1. Field of the Invention
The present invention relates an improvement in an assembly or construction of a polygon mirror for use, for example, in a laser scanner for reading a manuscript, in a laser printer, or in laser scanning apparatuses for measuring or examining.
2. Description of the Prior Art
A polygon mirror is such a mirror that is produced by making the side surfaces of a rotation body of polygon prism shape into mirror faces. A laser scanning is executed by rotating the polygon mirror by making a laser light to be reflected on the mirror surfaces.
The polygon mirror is required to have highly accurate dimensions and a high reflection coefficient, since the polygon mirror largely influences the quality of image output of the laser printer, etc. For satisfying such demand, generally the polygon mirror is produced by precisely polishing a glass surface and by evaporating a metal, for example, aluminum, on the polished glass surface. Alternatively, the polygon mirror is produced by processing a metal body of, for example, copper or aluminum material, utilizing a diamond member. The required accuracy is such that an allowable error an an inclination angle of the reflection surface of the polygon mirror against a reference face is .+-.5", a flatness of the reflection surface is 0.08 .mu.m and a roughness of the reflection surface is 0.01 to 0.02 .mu.m Ra.
Such high a precision polygon mirror is mounted on a shaft of a motor and is rotated by the motor. Therefore, the mounting mechanism of the polygon mirror, that is, the polygon mirror construction also should have high accuracy.
FIG. 12 shows a conventional polygon mirror construction. Two flanges or washers 32, 32 are fixed on an upper surface and on an lower surface of the polygon mirror 31 by an adhesive. The two flanges 32, 32 are held between a stopper 7 and a nut 5 which are mounted to the shaft 3 of the motor 2 with pressing the flanges 32, 32.
The polygon mirror 31 is rotated at 2000 to 3000 r.p.m., therefore the temperature of the motor 2 rises 60.degree. C. or higher. The raised temperature is transmitted to the flanges 32, 32, the stopper 7 and the nut 5, and further to the polygon mirror 31 via the shaft 3, thereby inducing the following problems.
(1) The dimension accuracy of the flanges 32, 32 is lost on account of heat expansion.
(2) The polygon mirror 31 is deformed or is destroyed by forced stress since the expansion coefficient of the polygon mirror 31 and the flange 32 are largely different and the polygon mirror 31 and the flanges are tightly fixed to each other by the adhesive.
(3) The accuracy of the polygon mirror construction is lowered by the heat expansion of the adhesive itself.
(4) The polygon mirror 31 is deformed on account of the heat.
As a result of the above, the reflection angle of the laser beam on the reflection surfaces of the polygon mirror 31 varies on account of the many problems and the image output quality of the laser printer decreases.